Schedule Constraints in the Devops Triangle

Last week’s post “The Devops Triangle” demonstrated the extension of Jim Highsmith‘s Agile Triangle to devops. The extension relied on adding compliance to the three traditional constraints of software development: scope, schedule, cost. A graphical representation of this extension is given in Figure 1.

Figure 1: Compliance as the Fourth Constraint in Devops Projects

This blog post examines how time/schedule should be governed in the devops context. It does so by building on the concluding observation in the previous post:

The Devops Triangle and the corresponding Tradeoff Matrix demonstrate how governance a la Agile can be extended to devops projects as far as compliance goes. The proposed governance framework however is incomplete in the following sense: schedule in devops projects can be a much more granular and stringent constraint than schedule in “dev only” projects.

For the schedule constraint in devops, I propose a schedule set. It consists of four components:

Lead Time or Engineering Time

Time to change

Time to deploy

Time to roll back

Lead Time/Engineering Time: These are customary metrics used in Kanban software development, as demonstrated in Figure 3.

Figure 3: The Engineering Time Metric Used by the BBC (David Joyce in the LSSC10 Conference)

﻿﻿Time to change: The amount of time it takes for the various stakeholders (e.g., dev, test, ops, customer support) to review the code to be deployed, approve its deployment and assign a time window for the deployment.

Time to deploy: The amount of time from (metaphorically speaking) pushing the Deploy “button” to completion of deployment.

Time to roll back: The amount of timeto undoa deployment. (Rigorous that the engineering practices and IT processes might be, the time to roll back a deployment can’t be ignored – it is a critical risk parameter).

A graphical representation of these four schedule metrics together with the Devops Triangle is given in the figure below:

Figure 4: The Devops Triangle with a Schedule Set

Using hours as the common unit of measure, a typical schedule set could be {100, 48, 3, 2}. In this hypothetical example, it takes a little over 4 days to carry out the development of the code increment; 2 days to get approval for the change; 3 hours to deploy the code; and, 2 hours to roll back.

Whatever your specific schedule numbers might be, it is highly recommended you apply value stream mapping (see Figure 5 below) to your schedule set. Based on the findings of the value stream mapping, apply statistical process control methods like those illustrated in Figure 3 to continuously improving both the mean and the variances of the four schedule components.